This invention relates to magnetrons, and more particularly to a magnetron in which a pair of permanent magnets are disposed with their same poles confronting each other.
In prior art magnetrons, permanent magnets were located outside the anode cylinder, and magnetic yokes were provided to form magnetic circuits together with the permanent magnets. In the magnetron of this kind, the amount of leakage flux was large, and the utilization efficiency of magnetic field was extremely low or only in the order of a few percents due to the fact that the permanent magnets were located remote from the interaction space defined between the cathode and the anode. For this reason, it was necessary to increase the size of the permanent magnets and yokes for the purpose of providing a magnetic field of required strength, in contradiction to the demands for reductions in the size and cost of the magnetron of this kind.
Recently, it has been proposed to dispose the permanent magnets within the tube of such a magnetron for the purpose of increasing the utilization efficiency of the magnetic field thereby decreasing the weights of the permanent magnets and yokes to attain the desired reductions in the size and cost.
However, there is an inevitable limitation in the material of permanent magnets to be disposed within the tube since magnets tending to liberate gases into the internal space of the tube are not fit for use. Practically utilizable materials for the permanent magnets have thus been limited to rare-earth-cobalt compounds, alnico and the like.
Such a magnetron, that is, a magnetron of the type having a pair of permanent magnets disposed within the tube is disclosed in, for example, U.S. Pat. No. 3,987,333. The disclosd magnetron comprises an anode cylinder of a magnetic material such as iron, a plurality of vanes secured to the inner wall of the anode cylinder to constitute a cavity resonator, and a cathode supported on the axial centerline of the anode cylinder. A pair of permanent magnets are disposed opposite to each other within the anode cylinder, and a pair of pole pieces are fixed to these permanent magnets respectively.
Therefore, the magnetic flux emanating from one of the permanent magnets passes through the pole piece fixed to that magnet to spread into the interaction space and then passes through the pole piece of the other magnet to enter this latter magnet. The magnetic flux entering this second magnet passes then through the anode cylinder to return to the first magnet.
In the magnetron of this kind, the interaction space required for producing the magnetic field is the space defined between the cathode and the anode. This interaction space is in the form of a cylinder having an inner diameter of about 5 mm and an outer diameter of about 10 mm, and it requires an axial length of about 8 to 10 mm although this axial length varies somewhat depending on the output of the magnetron.
The problem with the magnetic circuit in the magnetron of this kind is therefore how efficiently and inexpensively the required magnetic field can be produced in this interaction space. The magnetic field produced in this interaction space is required to have a strength, which should be varied depending upon the anode voltage, and a value of about 1,800 gauss is generally required for the anode voltage of 5 kilovolts. Further, it is required for the interaction space to produce a uniform magnetic field from the viewpoint of the stability of oscillation of the magnetron.
The structure of the cathode in the magnetron of this kind is generally classified into two types, and the structure of the permanent magnet and its pole piece located nearer to the cathode varies depending on the cathode structure.
In one cathode structure, a wide spacing is provided between the leads for the heater of the cathode, and a permanent magnet is disposed between these leads. In this case, the root portions of these two leads are superposed above the magnet with an insulator interposed therebetween, and a sufficient insulation distance must be provided between the vertically superposed root portions of the leads and the upper surface of the permanent magnet located nearer to the cathode. The above necessity results inevitably in the increase in the thickness of this part of the magnetron. Consequently, the gap length in the magnetic circuit is extended, and the actual interaction space is displaced upward relative to the center of the distance between the pole pieces of the upper and lower permanent magnets. That is, the upper portion of the interaction space approaches the upper permanent magnet, while the lower portion of the interaction space recedes from the lower permanent magnet. Therefore, non-uniformity occurs in the strength of the magnetic field produced in the interaction space, and it becomes necessary to increase the outer diameter of the lower permanent magnet in order to compensate for this non-uniformity of the field strength. Thus, even when a magnet of a rare-earth-cobalt compound is employed as this lower permanent magnet, the magnetron is more expensive than that using a strontium ferrite magnet presently commonly employed in this field.
In the other cathode structure, the cathode leads have a narrow spacing therebetween to be led to the exterior through the center of a permanent magnet, as shown in U.S. Pat. No. 3,987,333 cited hereinbefore. In this case, it is necessary to make the permanent magnet annular in shape. However, employment of such an annular permanent magnet affects adversely the magnetic field distribution along the inner peripheral portion of the interaction space, and the size of the specific permanent magnet and its pole piece must be increased to make uniform the magnetic field distribution in the interaction space. The volume of the specific permanent magnet is thus increased resulting in the increase in the cost of the magnetron.
It will be seen from the above discussion that how to rationally arrange the magnetic circuit in the vicinity of the permanent magnet located nearer to the cathode is an important problem in the magnetron of the kind above described.
The above description has referred to the employment of a rare-earth-cobalt magnet as the lower permanent magnet. If an alnico magnet were used as this lower permanent magnet, the small size which is the important advantage of the magnetron of this kind would be lost, since the alnico magnet having a small coercive force H.sub.c requires an elongated axial length. Further, it is required to increase the size of the anode cylinder from the viewpoint of minimizing leakage of the magnetic flux. This requires a structural arrangement including mounting of the vanes in spaced apart relation from the anode cylinder, resulting in complexity of the construction of the magnetron and in the increase in the cost of the magnetron.